Heat and light reflective coating on quartz lamp

ABSTRACT

A heat and light reflective coating on a fused silica envelope, as on the ends of a quartz arc tube in a metal halide lamp. The coating comprises a mixture of a reflective metal oxide such as zirconium oxide and 20 to 60 percent by weight of a finely ground up glass substantially free of alkali metal, particularly sodium. The coating is fired on at a temperature in the range from 1,100* to 1,200*C sufficient to soften the glass and cause the oxide particles to be bound to each other and to the quartz. The coatings are white, hard and very scratch resistant and able to withstand thermal cycling throughout lamp life.

States Patent [191 Thoasson 51 Nov. 26, 1974 HEAT AND LIGHT REFLECTIVECOATING 0N QUARTZ LAMP [21] Appl. No.: 313,875

3,374,377 3/1968 Cook 313/17 3,490,984 l/197O Petticrew et a1 106/52 X3,536,946 10/1957 Kopelman et ul 313/220 X 3,715,244 2/1973 Szupillo117/35 R X 3,754,980 8/1973 Malmendier 106/52 X Primary Examiner-AlfredL. Brody Attorney, Agent, or Firm--Ernest W. Legree; Lawrence R.Kompton; Frank L. Neuhauser [57] ABSTRACT A heat and light reflectivecoating on a fused silica envelope, as on the ends of a quartz arc tubein a metal halide lamp. The coating comprises a mixture of a reflectivemetal oxide such as zirconium oxide and 20 to 60 percent by weight of afinely ground up glass substantially free of alkali metal, particularlysodium. The coating is fired on at a temperature in the range from 1,100to 1,200C sufficient to soften the glass and cause the oxide particlesto be bound to each other and to the quartz. The coatings are white,hard and very scratch resistant and able to withstand thermal cyclingthroughout lamp life.

5 Claims, 1 Drawing Figure HEAT AND LIGHT REFLECTIVE COATING ON QUARTZLAMP BACKGROUND OF THE INVENTION The invention relates to heat and lightreflective coatings on quartz lamp envelopes and is particularly usefulin high intensity metal halide lamps having a high temperature quartzarc discharge tube enclosed in a larger glass envelope.

The metal halide lamps now in widespread use for industrial and outdoorlighting are disclosed in U.S. Pat. No. 3,234,421 Reiling, issued Feb.8, 1966, and entitled Metallic Halide Discharge Lamps. In appearance,these lamps resemble a conventional high pressure mercury vapor lampcomprising a quartz arc tube mounted within a glass outer jacketprovided with a screw base at one end. Therm'ionic electrodes aremounted in the ends of the arc tube which contains a quantity of mercuryand metal halides along with an inert gas for starting purposes. Onelamp in commercial production contains mercury, sodium iodide, thalousiodide and indium iodide, whereas another contains mercury, sodiumiodide, scandium iodide and thorium iodide.

The portions of the arc chamber behind the electrodes, that is the endsof the arc tube, are the coolest regions in normal operation of suchlamps. In the absence of special measures to raise the temperature ofthe ends, the metal halide such as sodium iodide rapidly condense on theenvelope wall behind the electrodes, making the lamp ineffective. Toprevent this, heat and light reflective coatings are generally appliedto the ends of the arc tube, sometimes to the lower end only invertically operated lamps. A coating which has been widely used isdescribed in US. Pat. No. 3,374,377 Cook, Metal Vapor Lamp Coating,issued Mar. 19, 1968 and consists essentially of zirconium oxide ZrOWhile the zirconium oxide coating has been quite satisfactory in respectof reflectivity and avoidance of darkening or release of deleteriousgases into the interenvelope space, it is quite fragile and will notwithstand abrasion. Bumping of lamps during handling may cause thecoating to flake off and this contributes to nonuniformity in color fromlamp to lamp and is an appearance defect.

SUMMARY OF THE INVENTION The object of the invention is to provide aheat and light reflective coating suitable for-use on quartz andquartz-like glasses such as are tubes of metal halide lamps, havingimproved adherence and resistance to abrasion and able to withstand thethermal cycling normat] to the operation of the lamp.

In accordance with my invention, I provide a coating comprising amixture of a reflective metal oxide and -60% by weight of a finelyground glass substantially free of alkali metal, particularly sodium,and having a softening point between 900-l ,200C. By substantially Ihave found experimentally that such coatings can be applied to quartz inthe range from l,l00 to l,200C. The coatings are white, hard and veryscratch resistant and able to withstand thermal cycling throughout lamplife.

Conventional lamp processing requires heat treatment of the arc tubes atabout l,200C in order to degas them, and this provides a convenientoccasion for application of the reflective coating. Thus. a glass whichstarts to soften below l,200C, but is not excessively reactive withquartz at this temperature is desirable. A preferred glass meeting theserequirements is that known as GE 177 comprising Si0 62.3 percent. A1 016.7 percent, BaO 18.8 percent, CaO 2.2 percent, and including less than0.05 percent alkali.

DESCRIPTION OF DRAWING The single FIGURE of the drawing is a side viewof a metal halide arc lamp embodying the invention in its arc tube.

DESCRIPTION OF PREFERRED EMBODIMENT An ideal coating would be one whichhas sufficient adherence to resist flaking during lamp life and which ishard and scratch resistant enough to withstand the normal bumps andabrasions received in lamp finishing operations. The coating must alsobe sufficiently refractory and thermal shock resistant to withstandrepeated cycling to temperatures of the order of 700C and it must notproduce any adverse effects on the quartz. The release of gas by thecoating during life must be negligible to avoid contamination of theouter jacket volume, and in particular release of gases which mightcontribute to are over at the mount must be avoided. Of course thecoating must be a good reflector of visible and infrared radiation inorder to perform its prime function.

I have found that desirable coatings can be made from a mixture ofreflective metal oxide and ground up glass which does not contain anyalkali or other materials which might induce quartz devitrification. Thepurpose of the glass is to bind the oxide particles to each other and tothe quartz arc tube and bonding is accomplished by heating the coatedarc tube to a high temperature in the range of about 1,100to l,200C fora time sufficient to soften the glass, a few minutes sufficing.

Glasses which I found satisfactory are GSC No. 4 quartz graded sealglass and GE I77 glass. The compo- Other glasses which may be used, andtheir compositions and properties are given in Table 2 below.

TABLE 2 Corning Corning O-l O-l Weight 1723 1717 EE2 EE SiO 56.3 66.561.5 63.7 A1 0 16.6 19.2 18.7 21.7 BaO 6.5 7.3 7.3 CaO 9.9 8 0 l 1.4 4.2B 0 3.3 MgO 8 2 3 2 Alkali 0.1 .06 .14 Properties Exp. Coef. 46 35 43 31X "/C Soft. PL, C 910 1107 955 1070 Anneal Pt.. C 710 861 761 819 StrainPL. C 670 804 714 772 Various coating compositions wherein theproportion of glass to metal oxide powder was varied from 20 to 60percent by weight and using either aluminum oxide or zirconium oxide forthe reflective oxide were made. The powders were combined with an ethylcellulose binder and ball milled. The coatings were then applied toquartz tubes by spraying, air drying, and then firing at temperatures of900, 1,000", l,100, and 1,200C for times ranging from 5 to minutes. The

best looking coatings were obtained with the 40 percent glass mixtureand these coatings were hard, white and very scratch resistant.

' Coatings were tested under normal lamp operating conditions in metalhalide lamps of otherwise conventional construction. As illustrated inthe drawing, such a lamp 1 comprises an outer glass envelope 2containing a quartz arc tube 3. The arc tube contains electrodes 4, 5set in opposite ends and has sealed therein a filling comprisingmercury, sodium iodide, thallium iodide, indium iodide and an inertstarting gas such as argon. The electrodes are connected to inlead 6, 7sealed through press 8 of stem 9 of outer envelope 2. The inleads areconnected externally to the contact surfaces of screw base 10 attachedto the neck end of the envelope.

The illustrated lamp is intended for base up operation and thereflective coating 11 has been applied to the lower end of the arc tubeonly. In a lamp intended for base down operation, the coating would beapplied to the opposite end of the arc 2. The outer envelope 2 may beevacuated as a heat conservation measure, or it may be filled with aninactive gas such as nitrogen. In the larger sizes of lamps exceeding400 watts, it is preferred to have a gas filling in the interenvelopespace.

Lamps with coated arc tubes were put on standard life tests to determineif there would be any detrimental effects on the quartz. Tests extendedto 4,000 hours life showed absolutely no change in the coating or in thequartz surface. I

Photometry tests indicated that when aluminum oxide was used for thereflecting oxide, generally higher coating reflectance and more heatinsulation was required. This could be achieved by applying heavierlayers but such heavy layers tend to crack and flake. The preferredsolution is to use zirconium oxide which has a higher refractive indexthan aluminum oxide for the reflective oxide. Also GB 177 glass ispreferred for the binder because of its low alkali content. For anygiven glass the melting point of-the glass sets an upper limit on thefiring temperature which cannot be exceeded withoutencounteringexcessive attack on the quartz by the glass. Such attackeventually results in crazing or cracking of the quartz. For instancewith the 40 percent GE 177 glass coating. a 1,200C firing temperaturedid not produce any quartz crazing. but in the case of the lower meltingGSC No. 4 glass, 1,200C was sufficiently high to cause excessivereaction with the quartz. However GSC No. 4 glass produces good resultswithout excessive reaction with the quartz when fired at 1,100C.-

Tests were run to determine whether there is an optimum particle sizefor the glass additive. Four different sieve fractions of GE 177 glasswere prepared consisting of 65-100 mesh, -150 mesh, 150-325 mesh, and325 mesh or less. By glass of 100-150 mesh is meant glass powder inwhich the particles can pass through a 100 mesh screen but not through a150 mesh screen. By far the best coatings were obtained with the finestglass powder fraction, that is the fraction passing through a 325 meshscreen. The average weight of the coatings applied in a test on 1,000watt arc tubes using ZrO and 40 percent by weight GE 177 glass was about370 milligrams per lamp. This means that about 222 milligrams of Zr0 wascoated on each lamp and this compared to about milligrams of ZrO when noglass binder is used as in US. Pat. No. 3,374,377 Cook. Thus myinvention requires but a moderate increase in the quantity of ZrO usedalong with the relatively inexpensive glass powder for the greatincrease in adherence, durability and scratch resistance achieved.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric lamp comprising a sealed quartz envelope containingradiation generating means,

and a hard, scratch-resistant heat and light reflective coating on aportion of the outside thereof, said coating comprising a reflectivemetal oxide powder having an index of refraction greater than 1.75, and20 to 60 percent by weight of finely ground up glass substantially freeof alkali metal,

the glass particles in said coating having been heatsoftened andre-fused to cause the metal oxide particles to be bound to each otherand to the quartz in a lightly sintered particulate coating.

2. A lamp as in claim 1 wherein said glass has a softening point between900 and 1,200C.

3. A lamp as in claim 11 wherein the reflective metal oxide powder iszirconium oxide.

4. A lamp as in claim 1 wherein the glass is composed of approximately62% SiO 17% A1 0 19% BaO, 2% CaO and not more than 0.05% alkali.

5. A lamp as in claim 1 wherein the glass is composed of approximately80.5% SiO 1.5% A1 0 17% B 0 and not more than 0.9% alkali.

1. AN ELECTRIC LAMP COMPRISING A SEALED QUARTZ ENVELOPE CONTAININGRADIATION GENERATING MEANS, AND A HARD, SCRATCH-RESISTANT HEAT AND LIGHTREFLECTIVE COATING ON A PORTION OF THE OUTSIDE THEREOF, SAID COATINGCOMPRISING A REFLECTIVE METAL OXIDE POWDER HAVING AN INDEX OF REFRACTIONGREATER THAN 1.75, AND 20 TO 60 PERCENT BY WEIGHT OF FINELY GROUND UPGLASS SUBSTANTIALLY FREE OF ALKALI METAL, THE GLASS PARTICLES IN SAIDCOATING HAVING BEEN HEAT-SOFTENED AND RE-FUSED TO CAUSE THE METAL OXIDEPARTICLES TO BE BOUND TO EACH OTHER AND TO THE QUARTZ IN A LIGHTLYSINTERED PARTICULATE COATING.
 2. A lamp as in claim 1 wherein said glasshas a softening point between 900* and 1,200*C.
 3. A lamp as in claim 1wherein the reflective metal oxide powder is zirconium oxide.
 4. A lampas in claim 1 wherein the glass is composed of approximately 62% SiO2,17% Al2O3, 19% BaO, 2% CaO and not more than 0.05% alkali.
 5. A lamp asin claim 1 wherein the glass is composed of approximately 80.5% SiO2,1.5% Al2O3, 17% B2O3 and not more than 0.9% alkali.